System and method for random access backoffs

ABSTRACT

A method of performing a random access procedure includes randomly selecting a backoff time from within a backoff window ranging from 0 to a specified multiple of a random access preamble unit, waiting until a time initialized with the backoff time expires, and retransmitting a random access preamble.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/160,776, filed on Oct. 15, 2018, entitled “System and Method forRandom Access Backoffs,” which is a continuation of U.S. patentapplication Ser. No. 15/659,909, filed on Jul. 26, 2017, entitled“System and Method for Random Access Backoffs,” (Now U.S. Pat. No.10,149,325 issued on Dec. 4, 2018), which is a continuation of U.S.patent application Ser. No. 15/451,983, filed on Mar. 7, 2017, entitled“System and Method for Random Access Backoffs,” which claims the benefitof U.S. Provisional Application No. 62/308,021, filed on Mar. 14, 2016,entitled “System and Method for Random Access Backoffs,” all of whichapplications are hereby incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The present invention relates generally to a system and method fordigital communications, and, in particular embodiments, to a system andmethod for random access backoffs.

BACKGROUND

When a user equipment (UE) initially attaches to a network orparticipates in a handover between cells, a random access procedure isperformed by the UE and the entity (such as an evolved NodeB (eNB), lowpower node (LPN), and so on) to which it is attaching in order to setupa connection with the entity.

SUMMARY

Example embodiments provide a system and method for random accessbackoffs.

In accordance with an example embodiment, a method for performing arandom access procedure is provided. The method includes randomlyselecting, by a user equipment (UE), a backoff time from within abackoff window ranging from 0 to a specified multiple of a random accesspreamble unit, waiting, by the UE, until a time initialized with thebackoff time expires, and retransmitting, by the UE, a random accesspreamble.

The specified multiple is one of a plurality of specified multiples, anddifferent specified multiples are selected for random access preambleswith different durations. There is a plurality of sets of specifiedmultiples, and the specified multiple is selected from one of theplurality of sets of specified multiples in accordance with a durationof the random access preamble. The random access preamble is initiallytransmitted on one of a first carrier or a first band, and the randomaccess preamble is retransmitted on one of a second carrier or a secondband.

The randomly selecting the backoff time includes selecting an initialbackoff time within a step of a predefined period, and selecting thebackoff time within the initial backoff time. The method also includessegmenting the random access preamble into a plurality of blocks,wherein retransmitting the random access preamble comprises separatelytransmitting each of the plurality of blocks. The separatelytransmitting each of the plurality of blocks includes interleaving atleast some of the plurality of blocks with an uplink data channel.

The random access preamble is transmitted in a network resource, andwherein the network resource also includes a gap inserted after thenetwork resource so that a duration of the network resource and a gaptime associated with the gap is equal to an integer multiple of asubframe duration.

In accordance with an example embodiment, a method for performing arandom access procedure is provided. The method includes determining, byan evolved NodeB (eNB), a backoff parameter value in accordance with arandom access preamble unit associated with a UE participating in therandom access procedure, signaling, by the eNB, an indicator of thebackoff parameter value, and receiving, by the eNB, a random accesspreamble in accordance with the backoff parameter value.

The backoff parameter value specifies a multiple of the random accesspreamble unit. There is a plurality of sets of specified multiples, andthe specified multiple is selected from one of the plurality of sets ofspecified multiples in accordance with a duration of the random accesspreamble.

The method also includes selecting a step of a predefined period, andsignaling an indicator of the step of the predefined period. The randomaccess preamble is segmented into a plurality of blocks, and receivingthe random access preamble includes separately receiving each of theplurality of blocks. The method also includes receiving an uplink datachannel interleaved with at least some of the plurality of blocks.

In accordance with an example embodiment, a method for transmitting arandom access preamble is provided. The method includes generating, by aUE, the random access preamble, and when a number of physical randomaccess channel (PRACH) repetitions per attempt is larger than athreshold, segmenting, by the UE, the random access preamble into aplurality of blocks, and separately transmitting, by the UE, each of theplurality of blocks. Separately transmitting each of the plurality ofblocks includes interleaving at least some of the plurality of blockswith an uplink data channel.

In accordance with an example embodiment, a non-transitorycomputer-readable medium storing programming for execution by at leastone processor is provided. The programming including instructions torandomly select a backoff time from within a backoff window ranging from0 to a specified multiple of a random access preamble unit, wait until atime initialized with the backoff time expires, and retransmit a randomaccess preamble.

The specified multiple is one of a plurality of specified multiples, andthe programming includes instructions to apply different specifiedmultiples for random access preambles of different durations. There is aplurality of sets of specified multiples, and the programming includesinstructions to select the specified multiple from one of the pluralityof sets of specified multiples in accordance with a duration of therandom access preamble. The programming includes instructions to selectan initial backoff time within a step of a predefined period, and selectthe backoff time within the initial backoff time. The programmingincludes instructions to segment the random access preamble into aplurality of blocks, and separately transmit each of the plurality ofblocks.

Practice of the foregoing embodiments enables the adaptation of thebackoff window used in contention resolution to meet the extendedpreamble durations of narrow band communications systems. Fixed backoffwindows cannot effectively deal with channel contention withoutsacrificing overall efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a high-level diagram of an Internet of Things (IoT)communications system;

FIG. 2 illustrates an example IoT communications system implementedusing a heterogeneous network (HetNet);

FIG. 3 illustrates an example random access procedure;

FIG. 4 illustrates a time-frequency diagram of a NB-PRACH transmissionwith 2 repetitions;

FIG. 5A illustrates a time-frequency diagram illustrating a collisionarising from an insufficiently small backoff parameter value;

FIG. 5B illustrates a diagram of network resources highlightingdifferent collision probabilities;

FIG. 6 illustrates an example backoff window that is 2 times the randomaccess preamble unit in duration according to example embodimentsdisclosed herein;

FIG. 7 illustrates a table of example backoff parameter values withdifferent sets of backoff parameter values assigned to different randomaccess preamble durations according to example embodiments disclosedherein;

FIG. 8A illustrates a time-frequency graph highlighting first exampleNB-PRACH operation in accordance with example embodiment 3;

FIG. 8B illustrates a time-frequency graph highlighting second exampleNB-PRACH operation in accordance with example embodiment 3;

FIG. 8C illustrates a time-frequency graph highlighting third exampleNB-PRACH operation in accordance with example embodiment 3;

FIG. 8D illustrates a time-frequency graph highlighting fourth exampleNB-PRACH operation in accordance with example embodiment 3;

FIG. 9A illustrates a first time-frequency graph highlighting amulti-step backoff in accordance with example embodiment 4;

FIG. 9B illustrates a second time-frequency graph highlighting amulti-step backoff in accordance with example embodiment 4;

FIGS. 10A and 10B illustrate example allocations of time-frequencyresources to random access channels according to example embodimentsdisclosed herein;

FIG. 11A illustrates a diagram of two bands or PRBs used formultiplexing channels of the same coverage level according to exampleembodiments disclosed herein;

FIG. 11B illustrates a diagram of two bands or PRBs when at least one ofthe bands or PRBs has unused resources according to example embodimentsdisclosed herein;

FIG. 11C illustrates a diagram of two bands or PRBs highlightingpriority based allocation according to example embodiments disclosedherein;

FIG. 11D illustrates a diagram of two bands or PRBs with separate bandor PRB allocation according to example embodiments disclosed herein;

FIG. 12 illustrates a time-frequency diagram highlighting the splittingof a long random access preamble into shorter parts according to exampleembodiments disclosed herein;

FIG. 13 illustrates a flow diagram of example operations occurring in aUE participating in a random access procedure according to exampleembodiments disclosed herein;

FIG. 14 illustrates a flow diagram of example operations occurring in aneNB participating in a random access procedure according to exampleembodiments disclosed herein;

FIG. 15 illustrates a diagram of network resources highlighting the useof a predefined gap time to align a random access preamble transmissionwith a 1 ms subframe boundary according to example embodiments disclosedherein;

FIG. 16A illustrates a first diagram of network resources highlighting acontinuous allocation of network resources according to exampleembodiments disclosed herein;

FIG. 16B illustrates a second diagram of network resources highlightinga mixed allocation of network resources according to example embodimentsdisclosed herein;

FIG. 17 illustrates a block diagram of an embodiment processing systemfor performing methods described herein; and

FIG. 18 illustrates a block diagram of a transceiver adapted to transmitand receive signaling over a telecommunications network.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently example embodiments are discussedin detail below. It should be appreciated, however, that the presentdisclosure provides many applicable inventive concepts that can beembodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the embodiments, and do not limit the scope of the disclosure.

FIG. 1 illustrates a high-level diagram of an Internet of Things (IoT)communications system 100. IoT communications system 100 includes anetwork 105 that is connected to a wide variety of IoT devices,including electrical devices, physical objects, and other items, thatare consumer, industrial, automotive, environment, agricultural,military, medical, and retail in nature. Network 105 allows the IoTdevices to be connected, as well as controlled. Network 105 may be anyexisting communications infrastructure, such as the Internet, a privateor public data network, and so on.

FIG. 2 illustrates an example IoT communications system 200 implementedusing a heterogeneous network (HetNet). The HetNet includes a plannednetwork infrastructure, such as a 3GPP LTE compliant communicationsnetwork, or any other standards or non-standards compliantcommunications network wherein communications take place throughentities that are deployed in a structured fashion. As shown in FIG. 2,the planned network infrastructure includes a plurality of evolvedNodeBs (eNBs), including eNB 205 with coverage area 206, eNB 207 withcoverage area 208, and eNB 209 with coverage area 210. The HetNet alsoincludes an unplanned network infrastructure. The unplanned networkinfrastructure may include low powered nodes (LPNs) that are deployed byan operator of the HetNet or by users of the HetNet to help improvecoverage and/or overall communications system performance. The unplannednetwork infrastructure shown in FIG. 2 includes a plurality of LPNs,including LPN 215 with coverage area 216 and LPN 217 with coverage area218. As their name implies, the LPNs usually transmit at a lower powerlevel than the eNBs of the planned network infrastructure. eNBs and LPNsmay be referred to as cells or transmission points.

The HetNet serves IoT devices, such as IoT device 220, IoT device 222,and IoT device 224, that are either mobile or immobile. The capabilitiesof the IoT devices may vary widely. As an example, if the IoT device isa smart device, such as a smart telephone, the IoT device may be capableof simultaneously communicating with multiple services, displaymultimedia, create multimedia, participate in an interactive session,serve data, and so on. As another example, if the IoT device is asensor, such as a security sensor or a weather temperature, the IoTdevice may be limited to periodically report its sensory reading to adata aggregator. Regardless of the capabilities of the IoT devices, theIoT devices need to be able to establish a connection with thecommunications infrastructure (e.g., the HetNet in FIG. 2).

eNBs are also commonly referred to as NodeBs, base stations,communications controllers, access points, and so on, depending on thetype of the planned network infrastructure. IoT devices are alsocommonly referred to as user equipments (UEs), mobile stations, mobiles,terminals, users, subscribers, stations, devices, smart devices, and soon, depending on the type of the IoT devices.

While it is understood that HetNets may employ multiple eNBs capable ofcommunicating with a number of IoT devices, only three eNBs, two LPNs,and three IoT devices are illustrated for simplicity.

As discussed previously, a random access procedure is performed by a UEwhen it initially attaches to a communications system or when itparticipates in a handover between cells. The UE participates in therandom access procedure with an entity (e.g., an eNB, a LPN, and so on)of the communications system or cell. In a Third Generation PartnershipProject (3GPP) Long Term Evolution (LTE) compliant communicationssystem, a random access procedure typically entails the UE selecting andtransmitting one out of a plurality of random access preambles to theeNB (an example of an entity of the communications system as describedabove) and the eNB assigns network resources to the UE to allow the UEto make a connection request. If the random access procedure fails, theUE must wait a certain amount of time before the UE can try again. Theamount of time that the UE waits is referred to as a backoff (BO).

FIG. 3 illustrates an example random access procedure 300. Random accessprocedure 300 involves processing performed by and transmissions made byan eNB 305 and a UE 310. Part of random access procedure 300 includes UE310 selecting a random access preamble and transmitting the randomaccess preamble to eNB 305. However, in situations when more than one UEis participating in a random access procedure, there is a non-zeroprobability that another UE will select the same random access preamble.When multiple UEs transmit the same random access preamble, a preamblecollision occurs (event 315). eNB 305 determines a backoff (BO)parameter (block 317) and transmits a random access response (RAR) withthe backoff parameter to UE 310 (event 319). UE 310 performs a backoffin accordance with the BO parameter provided by eNB 305 (block 321).Once the backoff completes, UE 310 transmits the random access parameter(event 323).

The RACH backoff mechanism used in 3GPP LTE is introduced in 3GPP LTERelease-13 and is based on media access control (MAC) backoff indication(BI). Limited changes to the backoff mechanism, such as a reduction inMAC BI size from 4 bits to another value and corresponding changes tobackoff range can be considered if needed and time permits. The UErandomly and uniformly chooses the backoff time from interval [0,backoff parameter value]. The interval [0, backoff parameter value] isalso referred to as the backoff window. The backoff parameter value issignaled in the form of an index into a table of backoff parametervalues. Table 1 shows Table 7.2-1, backoff parameter values from 3GPP TS36.321 and Table 2 shows 3GPP LTE Physical Random Access Channel (PRACH)preamble durations.

TABLE 1 3GPP TS 36.321Table 7.2-1 backoff parameter values. IndexBackoff Parameter value (ms) 0 0 1 10 2 20 3 30 4 40 5 60 6 80 7 120 8160 9 240 10 320 11 480 12 960 13 Reserved 14 Reserved 15 Reserved

TABLE 2 3GPP LTE PRACH preamble durations. Preamble Duration CP durationGuard time duration format (ms) (us) (us) 0 1 103.3 96.88 1 2 684.38515.63 2 2 203.13 196.88 3 3 684.38 715.63

In a narrow-band IoT (NB-IoT) ad hoc meeting and a RAN1 #84 meeting, thefollowing items were agreed upon:

-   -   An NB-PRACH scheme based on single-tone transmissions with 3.75        kHz subcarrier spacing is to be used in NB-IoT;    -   Each transmission consists of 4 groups with each group        comprising 1 cyclic prefix (CP) and 5 symbols in a symbol group;        and    -   1, 2, 4, 8, 16, 32, 64, and 128 NB-PRACH repetitions are        provided, with an eNB being able to configure up to 3 NB-PRACH        repetitions from the 8 provided repetitions. Table 3 shows        possible preamble durations based on the 8 provided repetitions.

TABLE 3 Possible preamble durations. Number of repetitions 1 2 4 8 16 3264 128 Preamble 6.4 = 12.8 25.6 51.2 102.4 204.8 409.6 819.2 length with1.6*4 266.7 us CP (ms) Preamble 5.6 = 11.2 22.4 44.8 89.6 179.2 358.4716.8 length with 1.4*4 66.7 us CP (ms)

However, these backoff parameter values and backoff time are not matchedwith NB-IoT preamble durations anymore. It can be seen that withrepetitions, NB-IoT PRACH preamble duration is even longer than some ofthe backoff parameter in Table 1, hence some small values make no sensecompared with longer preamble duration. For example, the backoffparameter value with toms is no use for preambles with more than onerepetition.

FIG. 4 illustrates a time-frequency diagram 400 of a NB-PRACHtransmission with 2 repetitions. As discussed previously, each NB-PRACHtransmission consists of 4 groups with each group comprising 1 CP and asymbol group with 5 symbols. As an illustrative example, a firstNB-PRACH transmission 405 includes 4 groups 410-416. As shown in FIG. 4,a second NB-PRACH transmission 420 also includes 4 groups (420-426) witheach group comprising 1 CP and a symbol group with 5 symbols.Additionally, pseudorandom frequency hopping is implemented betweenNB-PRACH transmissions, which is shown in FIG. 4 as frequency difference430 between first NB-PRACH transmission 405 and second NB-PRACHtransmission 420. However, the frequency resource usage within eachNB-PRACH transmission remains consistent between repetitions.

It is noted that compared to the 3GPP LTE PRACH preamble, the NB-PRACHpreamble duration can be much longer, especially in situations when alarge number of repetitions are transmitted for the purpose of coverageenhancement. Furthermore, there is also a much wider variation inNB-PRACH length. In some circumstances, e.g., with larger repetitionvalues, the NB-PRACH preamble duration is even longer than some of thebackoff parameter values currently used in 3GPP LTE. Hence, some of thesmaller backoff parameter values do not make practical sense whencompared with larger NB-PRACH preamble durations. As an illustrativeexample, a backoff parameter value of 10 ms is not useful for NB-PRACHpreambles with more than 1 repetition. The randomly selected backofftime does not match current NB-PRACH preamble durations. As anotherillustrative example, according to the 3GPP LTE backoff parametervalues, if a cell load is not heavy and a backoff parameter value isselected as 10 ms, but a random access preamble has a duration of 25.6ms (without gap time (GT)), the random access preamble will causeinterference. As used herein a gap time means a time duration when nosignals are transmitted. In a situation when inserted between a PhysicalUplink Shared Channel (PUSCH) and a PRACH in the time domain, a gap timeis the same as a guard time.

FIG. 5A illustrates a time-frequency diagram 500 illustrating acollision arising from an insufficiently small backoff parameter value.Collisions among RA preamble transmissions may happen due to themismatched backoff time and preamble durations. For discussion purposes,the situation illustrated in FIG. 5A involves NB-PRACH preambledurations of 25.6 ms and a backoff parameter value of toms. At event505, a first UE (UE1) and a second UE (UE2) transmit NB-PRACH preambles(preamble 508 for UE1 and preamble 509 for UE2) at the same networkresource, resulting in a preamble collision. Both UEs randomly selectbackoff times of 4 ms (for UE1) and 7 ms (for UE2), respectively.Therefore, after a backoff of 4 ms, UE1 retransmits its NB-PRACHpreamble (preamble 513) (event 510) and after a backoff of 7 ms, UE2retransmits its NB-PRACH preamble (preamble 514) (event 515). However,because the NB-PRACH preambles are 25.6 ms long, UE1 is unable tocomplete the transmission of its NB-PRACH preamble 513 before UE2transmits its NB-PRACH preamble 514 and another preamble collision takesplace.

Using the current backoff mechanism, the collision probability for longpreamble durations and short preamble durations differ greatly due tothe wide range of preamble lengths. As an illustrative example, underthe same cell load conditions and if the backoff parameter value is setto 960 ms, UEs using short preamble durations (e.g., 12.8 ms) will havemany more opportunities for access with low collision probability, butUEs using long preamble durations (e.g., 819.2 ms) will have lessopportunities for access with high collision probability. Additionally,due the narrow band nature of NB-IoT, long preamble durations may blockand cause extra delay in NB PUSCH (NB-PUSCH) transmission. Theallocation of network resources may need to be changed to reduce theblocking issue.

FIG. 5B illustrates a diagram 550 of network resources highlightingdifferent collision probabilities. Due to wide range of preamble length,given same resource unit, the collision probabilities for long preambleduration and short preamble duration differ significantly. Diagram 550displays three kinds of random access preamble durations, where each hasbeen allocated resources of the same size. With the legacy mechanism,for the same cell load, if the backoff window is fixed as a large onelike 960 ms, for UEs with short preamble duration like 12.8 ms preambleduration, they have more opportunities to access hence they may have lowcollision probability; while for UEs with long preamble duration like819.2 ms preamble duration, they have less opportunities and highcollision probability. It is noted that collision and interference maystill happen after backing off, since NB-PRACH preamble duration becomesmuch longer than some of the backoff parameter value defined for LTE,especially with large number of repetitions; the NB-PRACH preamblelengths vary widely and the collision probabilities may be different fordifferent preamble lengths in some cases.

According to an example embodiment, the backoff parameter values aredefined as multiples of random access preamble durations (random accesspreamble units) to align the backoff time with preamble duration.Instead of defining the backoff parameter values in time values, whichresults in widely varying access opportunities and collision probabilitywith different random access preamble durations, the defining of thebackoff parameter values as multiple of random access preamble unitsallows the access opportunities and collision probability to remainsubstantially constant with different random access preamble durations.Example embodiments include:

-   -   Example embodiment 1: The same set of backoff parameter values        is used for different random access preamble durations;    -   Example embodiment 2: Different sets of backoff parameter values        are used for different random access preamble durations or        repetitions;    -   Example embodiment 3: Backoff parameter values and frequency        hopping are combined to increase access opportunities and        decrease collision probabilities. It is noted that frequency        hopping may be performed within the same NB-PRACH band or        physical resource block (PRB) or in different NB-PRACH bands or        PRBs; and    -   Example embodiment 4: A multi-step backoff is performed,        -   Step 1—Backoff in number of a period, and        -   Step 2—Random offset within a period.            Detailed discussion of these example embodiments are            provided below.

According to an example embodiment, an eNB determines the backoffparameter values according to the load on corresponding random accesschannels. A technical standard or an operator of the communicationssystem may define possible backoff parameter values, such as a table ofpossible backoff parameter values. However, an eNB selects the actualbackoff parameter value(s) to signal to the UEs based on the load on therandom access channels.

According to an example embodiment, network resource allocation isperformed semi-statically to allocate the network resources for NB-PRACHbased on random access load. Different numbers of network resources maybe allocated for different NB-PRACH channels, with each random accesschannel being related to one NB-PRACH preamble format (e.g., duration,repetition, and so on). In order to reduce latency to the NB-PUSCH, longrandom access preambles may be split into multiple parts.

According to example embodiment 1, the backoff parameter values arespecified as multiples of random access preamble units and the samebackoff parameter values are used for different random access parameterdurations. The backoff window of a UE is defined as the product of thebackoff parameter value and the random access preamble unit, where therandom access preamble unit is equal to the random access preambleduration for the UE. Therefore, the random access window differs fordifferent UEs with different random access preamble durations. The useof the same backoff parameter values for different random accesspreamble durations is very simple with unified parameters. However, insituations with heavy loads, the latency for long random access preambledurations may be very large. The backoff parameter values may be definedby a technical standard or by an operator of the communications system.Table 4 shows an example backoff parameter value table. Table 5 showsexample backoff parameter values in ms and multiples of preambledurations.

TABLE 4 Example backoff parameter values. Backoff parameter values Index(number of preamble durations) 0 0 1 4 2 8 3 12 . . . . . . N 4N

TABLE 5 Example backoff parameter values in ms and multiples of preambledurations. Backoff parameter value Backoff parameter value Index (ms)(multiples of preamble durations) 0 0 0 1 256 40 2 512 80 3 1024 160 42048 320 5 4096 640 6 8192 1280 7 16384 2560 8 32768 5120 9 65536 1024010 131072 20480 11 262144 40960 12 524288 81920 13 Reserved Reserved 14Reserved Reserved 15 Reserved Reserved

The backoff window may be used in a manner same to the backoff window in3GPP LTE compliant communications systems, the UE randomly selects atime from within the backoff window and the UE waits the time expiresbefore retransmitting its NB-PRACH preamble.

The backoff window may be used in a manner similar to the backoff windowin 3GPP LTE compliant communications systems, the UE randomly selects anumber from within the backoff window and the UE waits the number timesthe random access preamble unit before retransmitting its NB-PRACHpreamble. FIG. 6 illustrates an example backoff window 600 that is 2times the random access preamble unit in duration. Table 6 shows examplebackoff times for two UEs with different random access preamble units.

TABLE 6 Example backoff times for two UEs with different random accesspreamble units. Preamble duration Backoff value (without GT) selected byUE Backoff time UE1: 12.8 ms 2 25.6 ms UE2: 25.6 ms 3 76.8 ms

According to example embodiment 2, backoff parameter values arespecified as multiples of random access preamble units and differentsets of backoff parameter values are used for different random accesspreamble repetitions. The use of different sets of backoff preamblevalues for different random access preamble repetitions enable theadjustment of different backoff window sizes for different random accesspreamble durations. As an example, long random access preamble durationsare assigned a set of backoff parameter values with small values, whileshort random access preamble durations are assigned a set of backoffparameter values with large values. FIG. 7 illustrates a table 700 ofexample backoff parameter values with different sets of backoffparameter values assigned to different random access preamble durations.As shown in FIG. 7, a first backoff parameter set 705 comprising smallerbackoff parameter values are assigned to high random access preamblerepetitions, a second backoff parameter set 710 comprising mediumbackoff parameter values are assigned to medium random access preamblerepetitions, and a third backoff parameter set 715 comprising a widerange of backoff parameter values are assigned to low random accesspreamble repetitions.

According to example embodiment 3, backoff parameter values arespecified as multiples of random access preamble units and a combinationof backoff window size and frequency hopping based on the number ofrandom access preamble repetitions is used. As an illustrative example,a non-zero backoff window size and frequency hopping are used insituations with small numbers of random access preamble repetitions. Asanother illustrative example, backoff windows are not used but frequencyhopping is used in situations with large numbers of random accesspreamble repetitions.

FIG. 8A illustrates a time-frequency graph Boo highlighting firstexample NB-PRACH operation in accordance with example embodiment 3. Asshown in FIG. 8A, the backoff window is a multiple of the random accesspreamble duration and the frequency hopping occurs within the samefrequency NB-PRACH band or PRB. In NB-IoT, a PRB may contain multipleNB-PRACH bands. FIG. 8B illustrates a time-frequency graph 820highlighting second example NB-PRACH operation in accordance withexample embodiment 3. As shown in FIG. 8B, the backoff window is amultiple of the random access preamble duration and the frequencyhopping occurs in a different frequency NB-PRACH band or PRB. Theexamples shown in FIGS. 8A and 8B may be preferably used in situationswith small numbers of random access preamble repetitions.

FIG. 8C illustrates a time-frequency graph 840 highlighting thirdexample NB-PRACH operation in accordance with example embodiment 3. Asshown in FIG. 8C, the backoff window is of duration zero (i.e., there isno backoff) and the frequency hopping occurs within the same frequencyNB-PRACH band or PRB. FIG. 8D illustrates a time-frequency graph 860highlighting fourth example NB-PRACH operation in accordance withexample embodiment 3. As shown in FIG. 8D, the backoff window is ofduration zero and the frequency hopping occurs in a different frequencyNB-PRACH band or PRB. The examples shown in FIGS. 8C and 8D may bepreferably used in situations with large numbers of random accesspreamble repetitions.

According to example embodiment 4, backoff parameter values arespecified as multiples of random access preamble units and a multi-stepbackoff is performed. As an illustrative example, in a situation whereinNB-PRACH resources are periodically allocated with each period includinga number of time-frequency resources allocated for random accesspreamble transmission, a two-step backoff includes: in a first step,backoff is performed in steps of a predefined period (the period andsteps of the period are determined by the eNB or UE); and in a secondstep, a random offset within the period is selected (the random offsetis determined by the UE). Frequency hopping can also be utilized.

FIG. 9A illustrates a first time-frequency graph 900 highlighting amulti-step backoff in accordance with example embodiment 4. As shown inFIG. 9, some time-frequency resources are allocated for random accesspreamble transmission, while others are allocated for PUSCHtransmission, gap time, and so on. Different UEs are assigned indifferent NB-PRACH periods. As an example, UE1 is assigned an NB-PRACHbackoff with one period meaning that UE1 can retransmit a NB-PRACHpreamble on a UE selected time-frequency resource in the next period,while UE2 is assigned an NB-PRACH backoff with two periods meaning thatUE2 can retransmit an NB-PRACH preamble on a UE selected time-frequencyresource in the second period following the first transmission. As shownin FIG. 9A, when the random access preamble is shorter, there are moretransmission opportunities, therefore the UEs can randomly select anaccess opportunity for random access preamble transmission, while forlonger random access preambles, there are fewer opportunities. FIG. 9Billustrates a second time-frequency graph 950 highlighting a multi-stepbackoff in accordance with example embodiment 4. As shown in FIG. 9B, noresources are allocated for PUSCH transmission, while others areallocated for random access preamble transmission, gap time, and so on.

In another example embodiment, it is beneficial to align NB-PRACH with 1millisecond subframe boundary of LTE. In one alternative, different gaptime can be inserted in the end of NB-PRACH. Table 7 lists example gaptimes that may be inserted at the end of a NB-PRACH preamble. The gaptimes listed in Table 7 are for illustrative purposes. The actual gaptime could be different and depends on the intended cell coverage and/orthe preamble duration. For example, the gap time for NB-PRACH with 128repetitions may be 0.8 ms for 266.7 us CP and 0.2 MS for 66.7 us CP,respectively. The gap time may be used for alignment purposes, such as a1 ms subframe boundary, for example.

TABLE 7 Example gap times. Number of repetitions 1 2 4 8 16 32 64 128Preamble 6.4 = 12.8 25.6 51.2 102.4 204.8 409.6 819.2 length with 1.6*4266.7 us CP (ms) Gap time (ms) 0.6 0.2 0.4 0.8 0.6 0.2 0.4 0.8 Preamble5.6 = 11.2 22.4 44.8 89.6 179.2 358.4 716.8 length with 1.4*4 66.7 us CP(ms) Gap time (ms) 0.4 0.8 0.6 0.2 0.4 0.8 0.6 0.2

In another alternative example embodiment, multiple NB-PRACH resourcescan be multiplexed in TDM. Each resource is dedicated to one type ofNB-PRACH with the same preamble duration. Multiple allocated NB-PRACHdurations are aggregated in time and followed by a gap time to align thewhole PRACH resource to 1 ms subframe boundary.

In another alternative example embodiment, multiple NB-PRACH resourcescan be multiplexed in TDM. Each resource is dedicated to one type ofNB-PRACH with the same preamble duration. Multiple allocated NB-PRACHdurations are aggregated in time to align the whole PRACH resource to 1ms subframe boundary. In this case, there is no gap time after PRACH.For example, 5 preambles with duration 6.4 ms can align with 1 mssubframe boundary.

According to an example embodiment, an eNB allocates the time-frequencyresources for NB-PRACH based on the random access load. The eNB may makeuse of the system information block (SIB) to signal the allocations. Asan illustrative example, the eNB may allocate different numbers oftime-frequency resources for different random access channels, with eachrandom access channel being related to a random access preamble format.The random access channels (i.e., the allocated time-frequencyresources) may be time division multiplexed (TDM) and/or frequencydivision multiplexed (FDM) within one NB-PRACH band or PRB.Alternatively, the random access channels may be allocated in differentNB-PRACH bands or PRBs. The load may be distributed on a random accesschannel basis rather than cell basis. When the load is high, moretime-frequency resources may be allocated.

FIGS. 10A and 10B illustrate example allocations of time-frequencyresources to random access channels. FIG. 10A illustrates an exampleallocation 1000 of time-frequency resources to random access channels ina FDM manner. FIG. 10B illustrates an example allocation 1010 oftime-frequency resources to random access channels in a TDM manner.

According to an example embodiment, the time-frequency resourcesallocated for random access channels are multiplexed with time-frequencyresources allocated for NB-PUSCH. The multiplexing of the NB-PUSCH andthe random access channels may help to reduce the latency of theNB-PUSCH cause by long NB-PRACH preambles.

According to an example embodiment, multiple bands or PRBs are used inmultiplexing the NB-PRACH and the NB-PUSCH. FIG. 11A illustrates adiagram 1100 of two bands or PRBs used for multiplexing channels of thesame coverage level. In a situation where the NB-PUSCH and the NB-PRACHare of the same coverage level (i.e., they have the same preambleduration), the channels are multiplexed into the same band or PRB, usingTDM, for example. FIG. 11B illustrates a diagram 1120 of two bands orPRBs when at least one of the bands or PRBs has unused resources. In asituation where there are unused resources in one or more bands or PRBs,the NB-PUSCH can be allocated the unused resources. FIG. 11C illustratesa diagram 1140 of two bands or PRBs highlighting priority basedallocation. In a situation with mixed multiplexing, the allocation ofthe resources may be made in accordance with the priority of thepreambles. As an example, long preambles have higher priority than shortpreambles. FIG. 11D illustrates a diagram 1160 of two bands or PRBs withseparate band or PRB allocation. The NB-PUSCH and the NB-PRACH may beallocated to separate bands or PRBs. If more than 2 bands or PRBs areavailable, each of the NB-PUSCH or the NB-PRACH may be allocated morethan 1 band or PRB.

According to an example embodiment, long random access preambles(preambles with long durations) are split into multiple parts. Each partmay be separately scheduled and transmitted. Reducing the duration ofthe random access preambles reduces the latency on the NB-PUSCH sincethe network resources are not allocated for extended amounts of time.Each of the shorter parts may utilize the backoff parameter valuesassigned to a whole random access preamble of the same duration as theshorter part. FIG. 12 illustrates a time-frequency diagram 1200highlighting the splitting of a long random access preamble into shorterparts. As shown in diagram 1200, a long random access preamble is splitinto 2 parts and is transmitted in resource 1205 and resource 1210. Itis noted that in order to align the boundaries (e.g., 2 ms slot or 4 mssubframe boundary), gap time may be inserted after a random accesspreamble.

FIG. 13 illustrates a flow diagram of example operations 1300 occurringin a UE participating in a random access procedure. Operations 1300 maybe indicative of operations occurring in a UE as the UE participates ina random access procedure.

Operations 1300 begin with the UE determining that the random accessprocedure has failed (block 1305). The random access procedure hasfailed if the eNB does not send a random access response to a randomaccess preamble sent by the UE, for example. Alternatively, the randomaccess procedure has failed if the UE receives a random access responsefrom the eNB, but the random access response is not for the UE butanother UE that sent the same random access preamble. The UE selects abackoff time from a backoff window ranging from [0, random accessparameter value*random access preamble unit] (block 1310). The randomaccess parameter value is signaled by the eNB. As an illustrativeexample, the eNB signals an indicator of which random access parametervalue to use out of a table of random access parameter values specifiedby a technical standard or an operator of the communications system. TheUE waits until the backoff time expires (block 1315). When the backofftime expires, the UE retransmits the random access preamble (block1320). In some example embodiments, the random access preamble issegmented into a plurality of blocks and the UE transmits each of theplurality of blocks. In some example embodiments, the UE interleaves aPUSCH with at least some of the blocks.

In a first aspect, the present application provides a method forperforming a random access procedure. The method includes randomlyselecting, by a UE, a backoff time from within a backoff window rangingfrom 0 to a specified multiple of a random access preamble unit,waiting, by the UE, until a time initialized with the backoff timeexpires, and retransmitting, by the UE, a random access preamble.

According to a first embodiment of the method according to the firstaspect, the specified multiple is one of a plurality of specifiedmultiples, and different specified multiples are selected for randomaccess preambles with different durations. According to a secondembodiment of the method according to any preceding embodiment of thefirst aspect or the first aspect as such, there is a plurality of setsof specified multiples, and the specified multiple is selected from oneof the plurality of sets of specified multiples in accordance with aduration of the random access preamble. According to a third embodimentof the method according to any preceding embodiment of the first aspector the first aspect as such, the random access preamble is initiallytransmitted on one of a first carrier or a first band, and the randomaccess preamble is retransmitted on one of a second carrier or a secondband.

According to a fourth embodiment of the method according to anypreceding embodiment of the first aspect or the first aspect as such,randomly selecting the backoff time includes selecting an initialbackoff time within a step of a predefined period, and selecting thebackoff time within the initial backoff time. According to a fifthembodiment of the method according to any preceding embodiment of thefirst aspect or the first aspect as such, the method also includessegmenting the random access preamble into a plurality of blocks, whereretransmitting the random access preamble includes separatelytransmitting each of the plurality of blocks. According to a sixthembodiment of the method according to any preceding embodiment of thefirst aspect or the first aspect as such, separately transmitting eachof the plurality of blocks includes interleaving at least some of theplurality of blocks with an uplink data channel. According to a seventhembodiment of the method according to any preceding embodiment of thefirst aspect or the first aspect as such, the random access preamble istransmitted in a network resource, and the network resource alsoincludes a gap inserted after the network resource so that a duration ofthe network resource and a gap time associated with the gap is equal toan integer multiple of a subframe duration.

In a second aspect, the present application provides a method fortransmitting a random access procedure. The method includes generating,by a UE, the random access preamble, and when a number of physicalrandom access channel (PRACH) repetitions per attempt is larger than athreshold, segmenting, by the UE, the random access preamble into aplurality of blocks, and separately transmitting, by the UE, each of theplurality of blocks.

According to a first embodiment of the method according to the secondaspect, separately transmitting each of the plurality of blocks includesinterleaving at least some of the plurality of blocks with an uplinkdata channel.

FIG. 14 illustrates a flow diagram of example operations 1400 occurringin an eNB participating in a random access procedure. Operations 1400may be indicative of operations occurring in an eNB as the eNBparticipates in a random access procedure.

Operations 1400 begin with the eNB determining a backoff parameter value(block 1405). The backoff parameter value is specified as a multiple ofa random access preamble unit (e.g., a random access preamble duration)of the UE. As an illustrative example, the same set of backoff parametervalues is used for all random access preamble units. As anotherillustrative example, different sets of backoff parameter values areused for different random access preamble durations. As yet anotherillustrative example, in addition to backoff parameter values based onrandom access parameter units, frequency hopping is also used. As yetanother illustrative example, a multi-step backoff is used, where aperiod for backoff is specified and an offset within the period iseither specified or is selectable by the UE. The eNB signals the backoffparameter value or an indicator thereof to the UE (block 1410). The eNBreceives a random access preamble in accordance with the backoffparameter value (block 1415).

In a third aspect, the present application provides a method forperforming a random access procedure. The method includes determining,by an eNB, a backoff parameter value in accordance with a random accesspreamble unit associated with a UE participating in the random accessprocedure, signaling, by the eNB, an indicator of the backoff parametervalue, and receiving, by the eNB, a random access preamble in accordancewith the backoff parameter value.

According to a first embodiment of the method according to the thirdaspect, the backoff parameter value specifies a multiple of the randomaccess preamble unit. According to a second embodiment of the methodaccording to any preceding embodiment of the third aspect or the thirdaspect as such, there is a plurality of sets of specified multiples, andthe specified multiple is selected from one of the plurality of sets ofspecified multiples in accordance with a duration of the random accesspreamble. According to a third embodiment of the method according to anypreceding embodiment of the third aspect or the third aspect as such,the method also includes selecting a step of a predefined period, andsignaling an indicator of the step of the predefined period.

According to a fourth embodiment of the method according to anypreceding embodiment of the third aspect or the third aspect as such,the random access preamble is segmented into a plurality of blocks, andreceiving the random access preamble includes separately receiving eachof the plurality of blocks. According to a fifth embodiment of themethod according to any preceding embodiment of the third aspect or thethird aspect as such, the method also includes receiving an uplink datachannel interleaved with at least some of the plurality of blocks.

In order to deal with the possible collision problem caused bymismatched backoff parameter values, it is to define the backoff time asmultiple times of basic time units, where the unit may equal to apreamble duration. From the RAN1 aspect, a problem is that the preambleduration (without considering GT) is not aligned with the subframeboundary of 1 ms subframe for 15 kHz subcarrier spacing and 4 mssubframe for 3.75 kHz subcarrier spacing, which may increase thescheduling complexity. According to an example embodiment, one solutionis to insert gap time. Following are two examples:

-   -   Example A: Append a variable length of gap time after the random        access preamble to align each random access preamble        transmission resource with subframe boundary. As shown above in        Table 3, the gaps between random access preamble durations and        subframe boundary are different. There may be four kinds of gap        time, e.g., {0.2 ms, 0.4 ms, 0.6 ms, 0.8 ms}, which can be used        to align the random access preamble with 1 ms subframe boundary.        However, some of them will result in unnecessary overhead.    -   Example B: Append a predefined length of gap time after a bundle        of random access preamble transmission resources to align a        bundle of random access preamble transmission resources with        subframe boundary. This approach is to multiplex the resources        for different preamble durations by TDM; a predefined gap time        (e.g., 0.2 ms) can be appended after a bundle of random access        preamble transmission resources. Hence the accumulated time can        be aligned with 1 ms subframe boundary. FIG. 15 illustrates a        diagram 1500 of network resources highlighting the use of a        predefined gap time to align a random access preamble        transmission with a 1 ms subframe boundary.

According to another example embodiment, one solution is to bundle aplurality of random access preamble transmissions without a gap time anduse a resource pattern to schedule the random access preambletransmissions. In this solution, it is assumed that there is a dedicatedband for random access transmissions, and eNB won't schedule NB-PUSCH inthis band. Hence there is no need for GT. Multiple random accesspreamble transmission resources corresponding to different preambledurations are multiplexed by TDM. FIG. 16A illustrates a first diagram1600 of network resources highlighting a continuous allocation ofnetwork resources. FIG. 16B illustrates a second diagram 1650 of networkresources highlighting a mixed allocation of network resources.

Proposal 1: Consider above solutions to align the PRACH channel withNB-IoT subframe boundaries. To deal with the possible unbalancedcollision probability for different PRACH preamble formats,semi-statically resource allocation for PRACH may be adopted. Firstly, arandom access channel load indicator can be defined instead of currentcell load. This is to say, the load is per random access channel basis.Each random access channel is related to one preamble format. When loadis heavy, more resources may be allocated. Consequently, eNB canallocate different number of resources for different random accesschannels based on random access channel load, in which the resourceallocation information may be carried via SIB.

Proposal 2: Consider semi-static resource allocation based on randomaccess channel load to balance the collision probabilities in differentrandom access channels for different preamble formats.

FIG. 17 illustrates a block diagram of an embodiment processing system1700 for performing methods described herein, which may be installed ina host device. As shown, the processing system 1700 includes a processor1704, a memory 1706, and interfaces 1710-1714, which may (or may not) bearranged as shown in FIG. 17. The processor 1704 may be any component orcollection of components adapted to perform computations and/or otherprocessing related tasks, and the memory 1706 may be any component orcollection of components adapted to store programming and/orinstructions for execution by the processor 1704. In an embodiment, thememory 1706 includes a non-transitory computer readable medium. Theinterfaces 1710, 1712, 1714 may be any component or collection ofcomponents that allow the processing system 1700 to communicate withother devices/components and/or a user. For example, one or more of theinterfaces 1710, 1712, 1714 may be adapted to communicate data, control,or management messages from the processor 1704 to applications installedon the host device and/or a remote device. As another example, one ormore of the interfaces 1710, 1712, 1714 may be adapted to allow a useror user device (e.g., personal computer (PC), etc.) tointeract/communicate with the processing system 1700. The processingsystem 1700 may include additional components not depicted in FIG. 17,such as long term storage (e.g., non-volatile memory, etc.).

In some embodiments, the processing system 1700 is included in a networkdevice that is accessing, or part otherwise of, a telecommunicationsnetwork. In one example, the processing system 1700 is in a network-sidedevice in a wireless or wireline telecommunications network, such as abase station, a relay station, a scheduler, a controller, a gateway, arouter, an applications server, or any other device in thetelecommunications network. In other embodiments, the processing system1700 is in a user-side device accessing a wireless or wirelinetelecommunications network, such as a mobile station, a user equipment(UE), a personal computer (PC), a tablet, a wearable communicationsdevice (e.g., a smartwatch, etc.), or any other device adapted to accessa telecommunications network.

In some embodiments, one or more of the interfaces 1710, 1712, 1714connects the processing system 1700 to a transceiver adapted to transmitand receive signaling over the telecommunications network. FIG. 18illustrates a block diagram of a transceiver 1800 adapted to transmitand receive signaling over a telecommunications network. The transceiver1800 may be installed in a host device. As shown, the transceiver 1800comprises a network-side interface 1802, a coupler 1804, a transmitter1806, a receiver 1808, a signal processor 1810, and a device-sideinterface 1812. The network-side interface 1802 may include anycomponent or collection of components adapted to transmit or receivesignaling over a wireless or wireline telecommunications network. Thecoupler 1804 may include any component or collection of componentsadapted to facilitate bi-directional communication over the network-sideinterface 1802. The transmitter 1806 may include any component orcollection of components (e.g., up-converter, power amplifier, etc.)adapted to convert a baseband signal into a modulated carrier signalsuitable for transmission over the network-side interface 1802. Thereceiver 1808 may include any component or collection of components(e.g., down-converter, low noise amplifier, etc.) adapted to convert acarrier signal received over the network-side interface 1802 into abaseband signal. The signal processor 1810 may include any component orcollection of components adapted to convert a baseband signal into adata signal suitable for communication over the device-side interface(s)1812, or vice-versa. The device-side interface(s) 1812 may include anycomponent or collection of components adapted to communicatedata-signals between the signal processor 1810 and components within thehost device (e.g., the processing system 1700, local area network (LAN)ports, etc.).

The transceiver 1800 may transmit and receive signaling over any type ofcommunications medium. In some embodiments, the transceiver 1800transmits and receives signaling over a wireless medium. For example,the transceiver 1800 may be a wireless transceiver adapted tocommunicate in accordance with a wireless telecommunications protocol,such as a cellular protocol (e.g., long-term evolution (LTE), etc.), awireless local area network (WLAN) protocol (e.g., Wi-Fi, etc.), or anyother type of wireless protocol (e.g., Bluetooth, near fieldcommunication (NFC), etc.). In such embodiments, the network-sideinterface 1802 comprises one or more antenna/radiating elements. Forexample, the network-side interface 1802 may include a single antenna,multiple separate antennas, or a multi-antenna array configured formulti-layer communication, e.g., single input multiple output (SIMO),multiple input single output (MISO), multiple input multiple output(MIMO), etc. In other embodiments, the transceiver 1600 transmits andreceives signaling over a wireline medium, e.g., twisted-pair cable,coaxial cable, optical fiber, etc. Specific processing systems and/ortransceivers may utilize all of the components shown, or only a subsetof the components, and levels of integration may vary from device todevice.

It should be appreciated that one or more steps of the embodimentmethods provided herein may be performed by corresponding units ormodules. For example, a signal may be transmitted by a transmitting unitor a transmitting module. A signal may be received by a receiving unitor a receiving module. A signal may be processed by a processing unit ora processing module. Other steps may be performed by a selectingunit/module, a waiting unit/module, a determining unit/module, and/or asignaling unit/module. The respective units/modules may be hardware,software, or a combination thereof. For instance, one or more of theunits/modules may be an integrated circuit, such as field programmablegate arrays (FPGAs) or application-specific integrated circuits (ASICs).

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims.

What is claimed is:
 1. A method for performing random access, the methodcomprises: receiving, by a user equipment (UE), a backoff indication;transmitting, by the UE, a first random access transmission of a randomaccess preamble to initiate a random access procedure; determining, bythe UE, that reception of a random access response corresponding to thefirst random access transmission of the random access preamble was notsuccessful; selecting, by the UE, a backoff parameter value from a setof backoff parameter values based on the backoff indication, the set ofbackoff parameter values consisting of multiples of a duration of therandom access preamble of the UE; and transmitting, by the UE, a secondrandom access transmission upon expiration of a backoff time followingthe determination that reception of the random access responsecorresponding to the first random access transmission of the randomaccess preamble was not successful, the backoff time being randomlyselected from between zero and the selected backoff parameter value. 2.The method of claim 1, wherein the set of backoff parameter values arestored by the UE.
 3. The method of claim 1, wherein the set of backoffparameter values consists of a zero multiple of the duration of therandom access preamble and a plurality of non-zero integer multiples ofthe duration of the random access preamble.
 4. The method of claim 3,wherein the set of backoff parameter values comprises twelve separatenon-zero integer multiples of the duration of the random accesspreamble.
 5. The method of claim 3, wherein the plurality of non-zerointeger multiples of the duration of the random access preamble includebackoff parameter values equal to 40 multiples of the duration of therandom access preamble, 80 multiples of the duration of the randomaccess preamble, 160 multiples of the duration of the random accesspreamble, 320 multiples of the duration of the random access preamble,640 multiples of the duration of the random access preamble, 1280multiples of the duration of the random access preamble, 2500 multiplesof the duration of the random access preamble, 5120 multiples of theduration of the random access preamble, 20,480 multiples of the durationof the random access preamble, 40,960 multiples of the duration of therandom access preamble, and 81,920 multiples of the duration of therandom access preamble.
 6. The method of claim 5, wherein the backoffparameter value indicated by the backoff indicator is set to one of thebackoff parameter values included in the plurality of non-zero integermultiples of the duration of the random access preamble.
 7. The methodof claim 1, wherein the second random access transmission is transmittedover a different carrier or band than the first random accesstransmission.
 8. A user equipment (UE) comprising: a processor; and anon-transitory computer readable storage medium storing programming forexecution by the processor, the programming including instructions to:receive a backoff indication; transmit a first random accesstransmission of a random access preamble to initiate a random accessprocedure; determine that reception of a random access responsecorresponding to the first random access transmission of the randomaccess preamble was not successful; select a backoff parameter valuefrom a set of backoff parameter values based on the backoff indication,the set of backoff parameter values consisting of multiples of aduration of the random access preamble of the UE; and transmit a secondrandom access transmission upon expiration of a backoff time followingthe determination that reception of the random access responsecorresponding to the first random access transmission of the randomaccess preamble was not successful, the backoff time being randomlyselected from between zero and the selected backoff parameter value. 9.The UE of claim 8, wherein the set of backoff parameter values arestored by the UE.
 10. The UE of claim 8, wherein the set of backoffparameter values consists of a zero multiple of the duration of therandom access preamble and a plurality of non-zero integer multiples ofthe duration of the random access preamble.
 11. The UE of claim 10,wherein the set of backoff parameter values comprises twelve separatenon-zero integer multiples of the duration of the random accesspreamble.
 12. The UE of claim 10, wherein the plurality of non-zerointeger multiples of the duration of the random access preamble includebackoff parameter values equal to 40 multiples of the duration of therandom access preamble, 80 multiples of the duration of the randomaccess preamble, 160 multiples of the duration of the random accesspreamble, 320 multiples of the duration of the random access preamble,640 multiples of the duration of the random access preamble, 1280multiples of the duration of the random access preamble, 2500 multiplesof the duration of the random access preamble, 5120 multiples of theduration of the random access preamble, 20,480 multiples of the durationof the random access preamble, 40,960 multiples of the duration of therandom access preamble, and 81,920 multiples of the duration of therandom access preamble.
 13. The UE of claim 10, wherein the backoffparameter value indicated by the backoff indicator is set to one of thebackoff parameter values included in the plurality of non-zero integermultiples of the duration of the random access preamble.
 14. The UE ofclaim 8, wherein the second random access transmission is transmittedover a different carrier or band than the first random accesstransmission.
 15. A method for facilitating random access, the methodcomprising: transmitting, by a base station, a backoff indicator to auser equipment (UE), the backoff indicator indicating a backoffparameter value in a set of backoff parameter values that consists ofinteger multiples of a duration of a random access preamble of the UE;and receiving, by the base station, a random access transmission fromthe UE, the random access transmission performed in accordance with thebackoff parameter value indicated by the backoff indicator.
 16. Themethod of claim 15, further comprising: adjusting, by the base station,the backoff indicator to indicate a different backoff parameter value inthe set of backoff parameter values based on a change in a random accesspreamble duration; and transmitting, by the base station, the adjustedbackoff indicator to the UE; and receiving, by the base station, asubsequent random access transmission from the UE, the subsequent randomaccess transmission performed in accordance with the backoff parametervalue indicated by the adjusted backoff indicator.
 17. The method ofclaim 16, wherein the subsequent random access transmission is receivedover a different carrier or band than the random access transmission.18. The method of claim 15, wherein the set of backoff parameter valuesconsists of a zero multiple of the duration of the random accesspreamble and a plurality of non-zero integer multiples of the durationof the random access preamble.
 19. The method of claim 18, wherein theset of backoff parameter values comprises twelve separate non-zerointeger multiples of the duration of the random access preamble.
 20. Themethod of claim 18, wherein the plurality of non-zero integer multiplesof the duration of the random access preamble include backoff parametervalues equal to 40 multiples of the duration of the random accesspreamble, 80 multiples of the duration of the random access preamble,160 multiples of the duration of the random access preamble, 320multiples of the duration of the random access preamble, 640 multiplesof the duration of the random access preamble, 1280 multiples of theduration of the random access preamble, 2500 multiples of the durationof the random access preamble, 5120 multiples of the duration of therandom access preamble, 20,480 multiples of the duration of the randomaccess preamble, 40,960 multiples of the duration of the random accesspreamble, and 81,920 multiples of the duration of the random accesspreamble.
 21. The method of claim 20, wherein the backoff parametervalue indicated by the backoff indicator is set to one of the backoffparameter values included in the plurality of non-zero integer multiplesof the duration of the random access preamble.
 22. A base stationcomprising: a processor; and a non-transitory computer readable storagemedium storing programming for execution by the processor, theprogramming including instructions to: transmit a backoff indicator to auser equipment (UE), the backoff indicator indicating a backoffparameter value in a set of backoff parameter values that consists ofinteger multiples of a duration of a random access preamble of the UE;and receive a random access transmission from the UE, the random accesstransmission performed in accordance with the backoff parameter valueindicated by the backoff indicator.
 23. The base station of claim 22,wherein the programming further includes instructions to: adjust thebackoff indicator to indicate a different backoff parameter value in theset of backoff parameter values based on a change in a random accesspreamble duration; and transmit the adjusted backoff indicator to theUE; and receive a subsequent random access transmission from the UE, thesubsequent random access transmission performed in accordance with thebackoff parameter value indicated by the adjusted backoff indicator. 24.The base station of claim 23, wherein the subsequent random accesstransmission is received over a different carrier or band than therandom access transmission.
 25. The base station of claim 22, whereinthe set of backoff parameter values consists of a zero multiple of theduration of the random access preamble and a plurality of non-zerointeger multiples of the duration of the random access preamble.
 26. Thebase station of claim 25, wherein the set of backoff parameter valuescomprises twelve separate non-zero integer multiples of the duration ofthe random access preamble.
 27. The base station of claim 25, whereinthe plurality of non-zero integer multiples of the duration of therandom access preamble include backoff parameter values equal to 40multiples of the duration of the random access preamble, 80 multiples ofthe duration of the random access preamble, 160 multiples of theduration of the random access preamble, 320 multiples of the duration ofthe random access preamble, 640 multiples of the duration of the randomaccess preamble, 1280 multiples of the duration of the random accesspreamble, 2500 multiples of the duration of the random access preamble,5120 multiples of the duration of the random access preamble, 20,480multiples of the duration of the random access preamble, 40,960multiples of the duration of the random access preamble, and 81,920multiples of the duration of the random access preamble.
 28. The basestation of claim 27, wherein the backoff parameter value indicated bythe backoff indicator is set to one of the backoff parameter valuesincluded in the plurality of non-zero integer multiples of the durationof the random access preamble.
 29. The method of claim 16, wherein therandom access transmission is a re-transmission of the random accesspreamble.
 30. The method of claim 1, wherein the UE receives the backoffindication after transmitting the first random access transmission ofthe random access preamble.
 31. The method of claim 1, wherein thesecond random access transmission includes the same random accesspreamble as the first random access transmission.